With the increased acceptance of and demand for electronic display and information systems, as in automobiles, systems designers have been confronted with a variety of problems. Some of these problems relate to providing data output in a form most useful to the viewer, and others relate simply to esthetic issues. One problem in particular, however, has presented a more serious issue because it touches upon both state and federal legal restrictions and requirements; the provision of an electronic odometer and trip odometer.
The task of the vehicle odometer and trip odometer can be described easily enough. The odometer must accurately measure and report the total distance the monitored vehicle has ever been driven. This information will be relied upon by maintenance personnel and by potential purchasers of the vehicle.
The trip odometer must accurately measure and report the total distance the monitored vehicle has been driven since a particular preselected starting point. This information will be relied upon by persons who are seeking to evaluate the vehicle's performance, maintaining maintenance schedules, or accumulating travel data. An odometer or trip odometer that fails to accurately measure travelled distance, or that fails to accurately report this distance, can contribute to incorrect decisions by these persons.
Current mechanical odometers and trip odometers meet these needs by providing geared members that turn in accordance with a calibrated schedule that relates to the distance being traveled by the vehicle. Simply stated, when the vehicle moves forward, the gears turn and the odometer count advances. When the vehicle does not move, the gears do not move and the odometer readout remains quiescent though viewable. The trip odometer differs from the odometer only in that the trip odometer may be easily reset to zero by the operator.
Fully electronic odometers and trip odometers are becoming available. Electronic displays for use with odometers and trip odometers are available, though such devices are not without problems. For instance, some sort of overflow indicia must be provided to indicated when the odometer reading has rolled over; i.e., that condition which results when the odometer display has exceeded its capacity and the odometer count has continued to advance. The prior art solution has been to provide a star or other overflow indicia that becomes a permanent part of the display when the odometer display rolls over. Unfortunately, other factors, such as strong radio frequency fields and the like, can ocassionally interfere with proper operation of an electronic odometer and cause the overflow indicia to be inadvertently set. Since this indicia becomes a permanent part of the display in prior art devices, the operator has no readily available means of correcting the display.
Further, an operator may wish to display the distance travelled by the vehicle in either English units or metric units. Though prior art devices allow such an option, there exists a possibility that a display in one unit of measurement would necessitate the provision of an overflow indicator, but not for another unit of measurement. An overflow indicator that becomes a permanent part of the display cannot readily accommodate such a need.
Trip odometer displays are also not without problem. Current prior art trip odometer displays operate by storing the trip start marker (i.e., the odometer reading at the start of the trip) in a non-volatile memory location. This trip marker may then be subtracted from later odometer readings in order to calculate and display the distance travelled during the trip. When the display capacity of the trip odometer has been met or exceeded, the prior art teaches that the original trip marker should be erased and a new trip marker should be automatically input and stored in the non-volatile memory.
There are two main problems associated with this prior art system. First, it is desirable to minimize erase/write events to the non-volatile memory, since such memories do have a finite lifetime. In addition, every erase/write event carries with it at least a small probability of error that may lead to inaccuracy, or, at worst, a disabling of the non-volatile memory device.
Second, prior art trip odometers fail to adequately resolve the situation that arises when the trip marker precedes a memory capacity rollover for the odometer. More particularly, all vehicle odometers have some upper limit with respect to the total distance that can be measured. If a trip marker precedes this upper limit, and the distance to be measured exceeds this point, then prior art techniques will provide an inaccurate indication of trip distance travelled.
There therefore exists a need for an incremental count display system that may be satisfactorily utilized in an odometer and trip odometer setting, and that will avoid the above indicated problems and limitations.